So far no one seems to have mentioned alternate air.
Quite a few installations bypass the air filter when carb heat is selected, so allowing the engine to breathe air directly, rather than through the slight restriction of the filter. If the outside air temperature is such that carb heat isn't going to change the inlet air temperature by much, then it's at least possible that the freer air flow resulting from bypassing the filter is allowing the small rpm increase observed.
It's worth remembering that selecting carb heat often bypasses the air filter, as it might just be useful should you experience a filter blockage in flight (flying through a sudden duststorm, perhaps?). The snag is that dust laden, unfiltered air, will fairly quickly result in excessive cylinder bore wear, but it might be a trick worth remembering if you ever get caught in something like a volcanic ash cloud (not likely, I know...............).